Imaging microviscometer

ABSTRACT

In one general aspect, a capillary viscometer is disclosed that includes a source of fluid pressure, a pressure transducer responsive to the source of fluid pressure, and a first capillary tube having an inside volume that is hydraulically responsive to the source of fluid pressure. A two-dimensional array of optical detectors is positioned proximate the first capillary tube with a first plurality of its detectors optically responsive to the inside volume of the first capillary tube and including an image data output. An acquisition driver circuit is responsive to the image data output of the two-dimensional array to acquire a series of successive images of the inside volume of the first capillary tube. Viscosity computation logic is responsive to the acquisition driver circuit and the pressure transducer, and operative to compute the viscosity of the fluid from pressure values received from the pressure transducer and the succession of images of the inside volume of the first capillary tube.

This application is a continuation-in-part of application Ser. No. 13/773,259, filed Feb. 21, 2013, which is herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for detecting properties of fluids, including viscosity, shear rate, and spectral characteristics.

BACKGROUND OF THE INVENTION

Lensless microfluidic detection techniques have been proposed to acquire microscopic images of samples such as biological materials and cells. They operate by acquiring images of suspended samples in close proximity to a high-resolution imaging detector. Their small size has resulted in their use being proposed in a variety of life science applications, including microscopes, smart petri dishes, and point-of-care diagnostic systems.

SUMMARY OF THE INVENTION

In one general aspect, the invention features a capillary viscometer that includes a source of fluid pressure, a pressure transducer responsive to the source of fluid pressure, and a first capillary tube having an inside volume that is hydraulically responsive to the source of fluid pressure. A two-dimensional array of optical detectors is positioned proximate the first capillary tube with a first plurality of its detectors optically responsive to the inside volume of the first capillary tube and including an image data output. An acquisition driver circuit is responsive to the image data output of the two-dimensional array to acquire a series of successive images of the inside volume of the first capillary tube. Viscosity computation logic is responsive to the acquisition driver circuit and the pressure transducer, and operative to compute the viscosity of the fluid from pressure values received from the pressure transducer and the succession of images of the inside volume of the first capillary tube.

In preferred embodiments, the viscosity computation logic can be responsive to the pressure transducer and be operative to compute a pressure-corrected viscosity. The apparatus can further include viscosity value selection logic responsive to the pressure transducer and with the viscosity value selection logic being operative to select a viscosity value based on detected pressure levels. The viscosity value selection logic can be operative to exclude outliers. The viscosity value selection logic can be operative to seek stable periods. The apparatus can further include a second capillary tube having an inside volume responsive to the source of fluid pressure, with the inside volume of the first capillary tube being larger than the inside volume of the second capillary tube, with the two-dimensional array of optical detectors also being positioned proximate the second capillary tube with a second plurality of its detectors optically responsive to the inside volume of the second capillary tube, and the apparatus can further include shear rate computation logic responsive to the acquisition driver circuit and operative to compute the viscosity of the fluid from the succession of images of the inside volume of the first and second capillary tubes. The apparatus can further include capillary tubes each having an inside volume responsive to the source of fluid pressure, with the inside volumes of the first capillary tube and the further tubes all being different from each other, with the two-dimensional array of optical detectors also being positioned proximate the further capillary tubes with further pluralities of its detectors each being optically responsive to the inside volumes of one the further capillary tubes, and the apparatus can further include shear rate computation logic responsive to the acquisition driver circuit and operative to compute a shear rate of the fluid from the succession of images of the inside volumes of the first capillary tube and the further capillary tubes. The first and further capillary tubes can be placed side-by-side where they are proximate the two-dimensional array of optical detectors, and are bundled at an open end. The viscosity computation logic can be operative to compute the viscosity based on detected movement of a meniscus in the first capillary tube. The viscosity computation logic can be operative to compute the viscosity based on a pixel size and frame rate. The apparatus can further include calibration storage for calibration information about a calibration run with a calibration standard, with the viscosity computation logic being responsive to the calibration information stored in the calibration storage. The apparatus can further include at least one calibration tube having an inside volume that is hydraulically responsive to at least one source of a known fluid standard with the two-dimensional array of optical detectors including a plurality of its detectors that are optically responsive to the inside volume of the calibration tube. The first capillary tube can have a diameter below about 500 microns. The two-dimensional array of optical detectors can be a two-dimensional array of visible light detectors. The two-dimensional array of optical detectors can be a two-dimensional array of infrared light detectors. The two-dimensional array of optical detectors can be a two-dimensional array of ultraviolet light detectors. The apparatus can further include a filter positioned in an optical path between the first capillary tube and the two-dimensional array of optical detectors. The filter is can be a variable filter, and the apparatus can further include spectrum derivation logic responsive to the two-dimensional array of optical detectors. The filter can be a bandpass filter.

In another general aspect, the invention features a capillary viscometry method, which includes driving a fluid under test through an inside volume of a first capillary tube, acquiring a succession of pressure values for the fluid under test, acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube, and deriving a viscosity of the fluid under test from the succession of pressure values and the successive acquired images of the fluid under test as it advances through the inside volume of the first capillary tube.

In preferred embodiments, the steps of acquiring pressure values and acquiring images can take place substantially synchronously. The method can further include the steps of driving the fluid under test through an inside volume of one or more further capillary tubes, acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube and the inside volumes of the further capillary tubes, and deriving a shear rate of the fluid from the succession of images of the inside volumes of the first capillary tube and the further capillary tubes. The step of deriving a viscosity is operative to derive a viscosity of a sample of less than 10 microliters. The method can further include the step of recovering the fluid under test from the first capillary tube after the step of acquiring. The method can further include the following calibration steps performed before the step of driving a fluid under test through the inside volume of the first capillary tube: driving a fluid calibration standard through an inside volume of a the capillary tube, acquiring successive images of the fluid calibration standard under test as it advances through the inside volume of the first capillary tube, and deriving calibration information from the successive acquired images of the fluid calibration standard as it advances through the inside volume of the first capillary tube, with the step of deriving a viscosity of the fluid under test deriving the viscosity of the fluid under test from calibration information and the successive acquired images of the fluid under test as it advances through the inside volume of the first capillary tube. The step of deriving a viscosity can be operative to derive a viscosity of a sample of less than 10 microliters. The method can further include the step of introducing a dye in the fluid under test, with the step of acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube being sensitive to the dye. The method can further include the step of deriving a spectrum of the fluid under test as it advances through the inside volume of the first capillary tube.

In a further general aspect, the invention features a capillary viscometer, which includes means for driving a fluid under test through an inside volume of a first capillary tube, means for acquiring a succession of pressure values for the fluid under test, means for acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube, and means for deriving a viscosity of the fluid under test from the succession of pressure values and the successive acquired images of the fluid under test as it advances through the inside volume of the first capillary tube.

Systems according to the invention can help to quickly characterize a variety of small samples of different fluid materials in research settings, such as in the discovery and manufacture of pharmaceuticals. They can also help to provide ongoing quality control and quality assurance in the manufacture of such materials.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of an embodiment of a fluid characterization system according to the invention, showing one capillary tube in phantom,

FIG. 2 is a block diagram of an embodiment of a high-throughput fluid characterization system according to the invention, and

FIG. 3 is flowchart showing an illustrative method of operation for the embodiment of FIG. 2.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Referring to FIG. 1, an embodiment of a fluid characterization system 10 according to the invention characterizes fluids that are pressurized by a pressure source 12. The pressure source receives fluid at a fluid input and is hydraulically connected to one or more capillary tubes 14 a . . . 14 n. The capillary tubes in this embodiment preferably have different diameters and are preferably laid parallel on a two-dimensional array detector 18, although it is possible to position them in other ways. By keeping the capillary tubes proximate the detector, no lens is needed in this embodiment, although a lens or lens system and/or other optical elements could be provided between the capillary tubes and the two-dimensional detector array.

In this embodiment, the pressure source 12 produces negative pressure to draw the fluid along the capillary tubes from one end. The fluid can then be collected and recovered, if desired, at the other end of the tube. But the pressure source can also be a positive or reversible pressure source, as described below in connection with FIG. 2.

An optional optical filter 20, such as a low-pass filter, a high-pass filter, a bandpass filter, or a variable filter, can be positioned between the capillary tubes 14 a . . . 14 n and the two-dimensional array detector 18. In the case of a variable filter, the filter can be made up of a series of areas 22 a . . . 22 m each having a corresponding different wavelength characteristic λa . . . λm. These can span the detector in a direction perpendicular to the flow axis of the capillary tubes. In one embodiment, the variable filter is built by coating the detector itself. The use of a variable filter can allow the system to acquire a spectrum of the fluid, such as an absorption or emission spectrum, in addition to the other information it acquires. The use of variable filters and detector arrays is discussed in more detail in U.S. Pat. No. 6,690,464, which is herein incorporated by reference.

An image acquisition module 30 is operatively connected to an image data output of the two-dimensional detector array. The image acquisition module is operatively connected to a fluid characteristics processing module 32, which can be connected to calibration storage 34. In this embodiment, a general-purpose computer 36 is operatively connected to the fluid characteristics processing module and the image processing module. It should be noted that these parts of the system can be built using special-purpose hardware, and/or software running on a general-purpose processing platform. In the case of a larger bench-top instrument, for example, much of the image acquisition and fluid characteristics processing can be performed by a standard computer workstation, such as a Windows®-based PC. In the case of a smaller stand-alone or hand-held system, however, more of the image acquisition and fluid characteristics processing could be performed with dedicated hardware.

One or more dedicated illumination sources 28 a . . . 28 y and illumination drivers 26 can also be provided, although separate lamps or even ambient light could be used in some circumstances. The illumination sources can provide high intensity light to improve signal-to-noise performance of the system. In the preferred embodiments, the illumination drivers can provide a strobed drive signal to the illumination sources to set an effective frame rate and thereby provide precise strobed detection times. Examples of strobed illumination sources can include pulsed LEDs or chopped lasers.

In operation of the illustrative fluid characterization system 10, the pressure source 12 draws a fluid under test through the one or more capillary tubes 14 a . . . 14 n. The image acquisition module 30 causes the two-dimensional array detector 18 to acquire a series of images of the capillaries tubes as the fluid advances through them. The images can be acquired in a number of different ways, depending on the architecture of the two-dimensional array detector 18, and whether strobing is employed. An array with a 500-1000 frame per second acquisition rate is currently believed to be well suited to this application although depending on the viscosity of the sample under test and the diameter of the capillary much lower frame rates will also work.

The fluid characteristic processing module 32 can first calculate the velocity at which the fluid passes through the capillary tubes 14 a . . . 14 n. This can be accomplished by multiplying the number of pixels advanced by the pixel pitch in the travel direction, and dividing the result by the time taken for that advance, which is equal to the number of frames multiplied by the frame rate. The reference used for this determination can be the meniscus of the fluid, although markers or other features could also be used. The velocity obtained for each tube can then be converted into viscosity based on one or more calibration factors obtained for one or more known fluid standards. Obtaining images that include samples and calibration standards is discussed in U.S. Pat. No. 7,391,025, which is herein incorporated by reference.

The system is preferably calibrated in one or more ways. One approach is to periodically run a known fluid standard with a known viscosity through the capillary tubes in the system, such as once a day or upon powering up the instrument. The resulting calibration factor compensates for offsets and drift, such as manufacturing variations in dimensions of the capillary tubes or drifts in temperature or power of the pressure source. Another approach is to provide a one or more dedicated capillary tubes for the calibration fluid so that a series of images that contain both the sample and one or more calibration standards are obtained. This approach may be more accurate than a periodic calibration, because the calibration and measurement are made in exactly the same conditions. Other known types of calibration can also be performed on the system, such as a flat field calibration; which helps to compensate for variations between detectors in the array. Calibration may be unnecessary in some circumstances, however, such as where only a fairly low accuracy is needed, or where relative measurements are more important than absolute measurements.

If the system is equipped with a single capillary tube or more than one capillary tube of the same diameter, it can be used as a simple viscometer that can quickly measure the viscosity of a small sample. If the system is equipped with multiple capillary tubes of different diameters, the system can calculate viscosity values for each of these tubes. This allows the system to report a range of viscosity values, which can provide insight into non-Newtonian effects. In one embodiment, the viscosity values or derived shear values are presented in a plot against tube diameter, although they can be supplied as raw numbers or input as parameters in a more complex mathematical model. The tube diameter can be obtained from the manufacturer, it can be derived from its size in acquired images, or it can be determined in other ways.

The system should preferably also include temperature monitoring and/or control. Because viscosity is temperature dependent, measurements should preferably be performed at a predetermined temperature, or that the temperature be known, so that it can be compensated for. For this reason, a temperature control module 38 is provided in the system of FIG. 1. This control module ensures that the fluid under test and the measurement capillary tubes are all kept at the same predetermined temperature. In addition, the viscosity of the sample at different temperatures may be required to be determined and the same temperature control system can be used to accomplish this.

Different types and sizes of capillary tubes can be used. In one embodiment, off-the-shelf capillary electrophoresis tubing is used, with inside diameters are on the order of 75 microns. The diameter of the capillary tubing is presently contemplated as being uniform along the length of the tubing, although it would also be possible to vary the diameter, such as by cascading different diameters of tubes to obtain serial measurements.

Referring to FIG. 2, an embodiment of a high-throughput fluid characterization system 40 according to the invention uses a probe 42 to perform successive measurements on a number of liquid samples held in different vessels, such as wells 46 of a multi-well plate 44 or carousel. The pressure source in this case is a reversible pressure source 12 that is hydraulically connected to one or more capillary tubes tubes 14 a . . . 14 n via a manifold 48. The capillary tubes are laid parallel on a two-dimensional array detector 18, although it is possible to position them in other ways. One or more of the tubes might snake back and forth in front of the detector, for example, if there is room. The ends of the capillary tubes can also be bundled together at their open ends to make the probe tip more compact and thereby fit into small sample vessels.

In this embodiment, an off-the-shelf x-y-z stage is provided to successively position the samples under the probe, although other types of mechanisms can be used to position the vessels and probe relative to each other. Wash and waste vessels can also be provided, either in the plate, or separately. Analysis and control logic, as described in connection with the system described in FIG. 1, can also be provided.

Referring also to FIG. 3, in an illustrative operation sequence 50, the x-y-z stage begins by positioning a first of the wells below the probe to select a first sample (step 52). The image acquisition module then acquires an image of the first sample (step 54). If no meniscus is detected in any of the capillary tubes (step 56), the system 40 continues to acquire images until a meniscus is detected in at least one of the capillary tubes.

When a meniscus is detected in one or more of the tubes, its position relative to the detectors in the array is recorded (step 58). Further images are then acquired (step 60) until no more meniscuses are detected in the images (step 62). The fluid characteristic processing module can then derive results for the first sample, such as one or more velocity, one or more viscosity, one or more spectrum, and/or one or more shear value (step 64).

The first sample can then be returned to the first well or it can be discarded into a waste well, and the probe can be washed in a wash well (step 66). If there are more samples to process (step 68), the x-y-z stage can select the next sample in the sequence (step 70). The process can then be repeated until there are no further samples to be tested, or some other condition is reached.

One of ordinary skill the art will recognize that there are a variety of ways to vary the illustrative operation sequence presented in connection with FIG. 3 without departing from the scope and spirit of the invention. The system might simply repeatedly acquire a series of images while the pump is on, and then look for images in which there is a meniscus in the resulting set of images. Or the system might acquire and store only a subset of the images, such as the first and last meniscus images for each capillary tube, and then process them all after the acquisition is complete for the whole set of samples.

The velocities can also be calculated a number of different ways. The velocity can be derived from time taken to span the whole detector, for example. Or instantaneous velocities in each successive frame pair could be calculated and then averaged. It is also possible to vary the pressure produced by the pump, such by steadily increasing the pressure in a ramp profile, and to then acquire a velocity profile instead of a single velocity for each capillary tube. The instantaneous meniscus velocity can also be monitored to ensure that the fluid is moving steadily, or it can even be used in a feedback loop to govern the speed of the pressure source.

Systems and methods according to the invention are particularly well suited to characterizing sample materials such as biomaterials, biopharmaceuticals or pharmaceutical formulations. A 96-well plate that holds a number of very small samples of biopharmaceutical candidate materials, for example, can be characterized to quickly narrow the field of materials under consideration. Or vessels can be loaded with one material diluted to different concentrations and/or held at different pH levels to more fully characterize that material. This can be particularly helpful where high viscosities in end products might prevent them from being injected and/or pumped.

A fluid characterization system according to the invention can also include a pressure sensor 80 that is responsive to the pressure produced by the pressure source 12, and can provide its output to the fluid characteristics processing module 32. This pressure sensor can be used in a feedback loop to keep the pressure in the system steady. The pressure sensor can also sample the pressure in synchrony with the acquisition of images and use the pressure measurements to correct the viscosity measurements for fluctuations in pressure.

Another alternative is to use the pressure measurements to determine the quality of data being acquired and eliminate samples that are associated with outlying values. In an illustrative set of samples 82, for example, the system can determine whether samples fall below a preset minimum threshold P− or above a preset maximum threshold P+ and refrain from using data from the images acquired during those excursions (e.g., samples 13-17). Other monitoring methods are also possible. For example, the system can monitor the derivative of pressure and choose the most stable period or set of periods to perform viscosity measurements. The monitoring and discarding of samples can take place in real time or it can be performed on an acquired data set that includes a series of images and corresponding pressure values. Sampled temperature values can be included as well to provide a three-dimensional data set that can be corrected for pressure and temperature variations. In another embodiment, the acquisition of images can be gated by the pressure sensor, so that the system effectively waits for a stable period before performing a viscosity measurement.

The present invention has now been described in connection with a number of specific embodiments thereof. However, numerous modifications which are contemplated as falling within the scope of the present invention should now be apparent to those skilled in the art. For example, a multi-well probe could be devised to acquire samples from two or more wells at a time. Therefore, it is intended that the scope of the present invention be limited only by the scope of the claims appended hereto. In addition, the order of presentation of the claims should not be construed to limit the scope of any particular term in the claims. 

What is claimed is:
 1. A capillary viscometer, comprising: a source of fluid pressure, a pressure transducer responsive to the source of fluid pressure, a first capillary tube having an inside volume that is hydraulically responsive to the source of fluid pressure, a two-dimensional array of optical detectors positioned proximate the first capillary tube with a first plurality of its detectors optically responsive to the inside volume of the first capillary tube and including an image data output, an acquisition driver circuit responsive to the image data output of the two-dimensional array to acquire a series of successive images of the inside volume of the first capillary tube, viscosity computation logic responsive to the acquisition driver circuit and the pressure transducer, and operative to compute the viscosity of the fluid from pressure values received from the pressure transducer and the succession of images of the inside volume of the first capillary tube.
 2. The apparatus of claim 1 wherein the viscosity computation logic is responsive to the pressure transducer and is operative to compute a pressure-corrected viscosity.
 3. The apparatus of claim 1 further including viscosity value selection logic responsive to the pressure transducer and wherein the viscosity value selection logic is operative to select a viscosity value based on detected pressure levels.
 4. The apparatus of claim 3 wherein the viscosity value selection logic is operative to exclude outliers.
 5. The apparatus of claim 3 wherein the viscosity value selection logic is operative to seek stable periods.
 6. The apparatus of claim 1, further including a second capillary tube having an inside volume responsive to the source of fluid pressure, wherein the inside volume of the first capillary tube is larger than the inside volume of the second capillary tube, wherein the two-dimensional array of optical detectors is also positioned proximate the second capillary tube with a second plurality of its detectors optically responsive to the inside volume of the second capillary tube, and further including shear rate computation logic responsive to the acquisition driver circuit and operative to compute the viscosity of the fluid from the succession of images of the inside volume of the first and second capillary tubes.
 7. The apparatus of claim 1, further including further capillary tubes each having an inside volume responsive to the source of fluid pressure, wherein the inside volumes of the first capillary tube and the further tubes are all different from each other, wherein the two-dimensional array of optical detectors is also positioned proximate the further capillary tubes with further pluralities of its detectors each being optically responsive to the inside volumes of one the further capillary tubes, and further including shear rate computation logic responsive to the acquisition driver circuit and operative to compute a shear rate of the fluid from the succession of images of the inside volumes of the first capillary tube and the further capillary tubes.
 8. The apparatus of claim 7 wherein the first and further capillary tubes are placed side-by-side where they are proximate the two-dimensional array of optical detectors, and are bundled at an open end.
 9. The apparatus of claim 1 wherein the viscosity computation logic is operative to compute the viscosity based on detected movement of a meniscus in the first capillary tube.
 10. The apparatus of claim 1 wherein the viscosity computation logic is operative to compute the viscosity based on a pixel size and frame rate.
 11. The apparatus of claim 1 further including calibration storage for calibration information about a calibration run with a calibration standard, and wherein the viscosity computation logic is responsive to the calibration information stored in the calibration storage.
 12. The apparatus of claim 1 further including at least one calibration tube having an inside volume that is hydraulically responsive to at least one source of a known fluid standard and wherein the two-dimensional array of optical detectors includes a plurality of its detectors that are optically responsive to the inside volume of the calibration tube.
 13. The apparatus of claim 1 wherein the first capillary tube has a diameter below about 500 microns.
 14. The apparatus of claim 1 wherein the two-dimensional array of optical detectors is a two-dimensional array of visible light detectors.
 15. The apparatus of claim 1 wherein the two-dimensional array of optical detectors is a two-dimensional array of infrared light detectors.
 16. The apparatus of claim 1 wherein the two-dimensional array of optical detectors is a two-dimensional array of ultraviolet light detectors.
 17. The apparatus of claim 1 further including a filter positioned in an optical path between the first capillary tube and the two-dimensional array of optical detectors.
 18. The apparatus of claim 17 wherein the filter is a variable filter, and further including spectrum derivation logic responsive to the two-dimensional array of optical detectors.
 19. The apparatus of claim 17 wherein the filter is a bandpass filter.
 20. A capillary viscometry method, comprising: driving a fluid under test through an inside volume of a first capillary tube, acquiring a succession of pressure values for the fluid under test, acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube, and deriving a viscosity of the fluid under test from the succession of pressure values and the successive acquired images of the fluid under test as it advances through the inside volume of the first capillary tube.
 21. The method of claim 20 wherein the steps of acquiring pressure values and acquiring images take place substantially synchronously.
 22. The method of claim 20 wherein the step of deriving a viscosity is operative to derive a viscosity of a sample of less than 10 microliters.
 23. The method of claim 20 further including the step of recovering the fluid under test from the first capillary tube after the step of acquiring.
 24. The method of claim 20 further including the following calibration steps performed before the step of driving a fluid under test through the inside volume of the first capillary tube: driving a fluid calibration standard through an inside volume of a the capillary tube, acquiring successive images of the fluid calibration standard under test as it advances through the inside volume of the first capillary tube, and deriving calibration information from the successive acquired images of the fluid calibration standard as it advances through the inside volume of the first capillary tube, and wherein the step of deriving a viscosity of the fluid under test derives the viscosity of the fluid under test from calibration information and the successive acquired images of the fluid under test as it advances through the inside volume of the first capillary tube.
 25. The method of claim 20 wherein the step of deriving a viscosity is operative to derive a viscosity of a sample of less than 10 microliters.
 26. The method of claim 20 further including the step of introducing a dye in the fluid under test, and wherein the step of acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube is sensitive to the dye.
 27. The method of claim 20 further including the step of deriving a spectrum of the fluid under test as it advances through the inside volume of the first capillary tube.
 28. A capillary viscometer, comprising: means for driving a fluid under test through an inside volume of a first capillary tube, means for acquiring a succession of pressure values for the fluid under test, means for acquiring successive images of the fluid under test as it advances through the inside volume of the first capillary tube, and means for deriving a viscosity of the fluid under test from the succession of pressure values and the successive acquired images of the fluid under test as it advances through the inside volume of the first capillary tube. 